High Efficiency Boilers

I have a 4000 square ft living space with a 30 year old Boiler in Michigan. The home is a Brick colonial built in 1948, with an addition built in 1978. I don't believe the walls in the older section are insulated, and it is likely to be of a balloon framing.
My natural gas bills during winter are very high and I would like to slash them by investing in a Very High Efficiency boiler. I have measured and totalled the number of radiators and baseboard heaters.

I also want to heat my hot water with this boiler, however, I have been told that I should have a storage tank. My understanding was that Buderus and Baxi provide instant hot water and I would not need a tank. These add on storage tanks are very expensive and my father in-law suggests that I keep my current gas water heater, but insert a stripped down, unpowered electric water heater in-line, ahead of my water heater so I could prewarm supply water to the tank to room temp and thereby save money.
Trouble is, I don't know which boiler/setup is best for my home?

My father works as an electrician and has been impressed by Viessman and Buderus.

My local plumber recommends Baxi bc it fully modulates something...

Another guy seems to only installs Knight/Lochinvar

I figure someone here truly knows the best device. Key points for me are, if it breaks can I get parts?

Will the company be around for sometime to come?

Are standard parts compatible? (I understand Lennox does not have standard parts or fittings)

So you're going to choose the plumber by which mod/conboiler you decide to go withÂ¿ A homeowner can go with whichever they choose but we will, 'til we're blue in the face, steer you in the direction of Lochinvars's Knight boilers.
I would say do the research, call up some reps after you have some knowledge to work with and then hopefully the plumber/mechanic you have designing the hydronic system agrees with a boiler you find appropriate.

Some thoughts:
you will save more money by insulating than by a high efficiency heating system.

A tripped down old electric water heater as tempering tank: you will convert the heated air next to the tank into heated water.
No net gain or loss, since the money you spend to heat the air is now going instead to heat the water.

Tank less water heaters are not storage tanks, are very expensive, and not cheap to run either, I see little gained there.

I have a Buderus at my home. It's been in a couple of seasons now. I have an indirect WH (a priority zone off the boiler) that works well at filling a big 6' tub with other simultaneous uses. It can modulate down to as low as 20% of max to match the heating load. There is almost no volume of water in the boiler so response is quite fast. The US distributor is about 15-miles from my house in NH. I've not needed any service or parts, and as with any system, the quality and knowledge of the installer is very important to getting a system up and running efficiently.

There is another boiler out there that also is a high efficiency, fully modulating, wall-mounted boiler. It is called the LAARS Mascot and is available in either heat only or "combi" (heat and domestic hot water).
It is similar to the Baxi boiler. They run around 96% efficient.

My opinion would be to insulate the home better first, then install a new boiler/system if need be. That way you can get the most out of your investments. It is all about the return on your investment.

Do the research and find out which is best for you. Get several opinions.

But a PROPERLY sized unit will beat the socks off a properly sized cast iron beast. And a properly-sized unit will be smaller, and cost less up front. t's common to see them 3-4x oversized, which makes a 78% AFUE boiler run at a true 60-65%, or a 90% condensing boiler at a mere, 80%, etc. If anything, undersize it a bit relative to the Manual-J calculation (20% smaller is usually safe enough.) If you actually get cold after having done that it's usually because there are some easy/cheap to fix heat leaks (insulation gaps, air-infiltration paths that can be sealed, etc.)

If you're radiation (radiators/baseboards/radiant floors, etc) was designed to run at 180F on design-day (coldest hours of the year) and you actually RUN them there, you won't get the condensing benefit of the modulating-condensing boilers, but you'll get some if you utilize "outdoor reset" controls.

An indirect-fired tank will...

A: ...be more efficient than running the mod-con boiler as a tankless, since the boiler won't see as many cycling losses, and...

B:...will keep the boiler's ignition & control systems from a premature death, since it won't cycling on/off as often and...

C.... will even outlast the boiler itself- it'll be the last hot water heater you'll ever buy.

The Baxi Luna has a bad rep, but I don't have any direct experience with them (maybe they've fixed their problems... or maybe not?.)

There are many good mod-cons out there. The Triangle Tube Solo series has a good rep and getting better. Peerless Pinnacles too.

First time installation costs on a mod-con are substantially greater than for a cast-iron or steel replacement, as is the boiler itself. If you don't have sufficient radiation to get the heat out at low temp you may get better (and permanent) payback by insulating the place, if it's the typical brick-veneer/cavity/un-insulated studwall that was prevalent in the 1940s. Most insulation installers can pound cellulose or loose fiberglass into the cavity wall without creating other problems, but get references before going that route.

Here, I'll get my 2 cents in. If you have old cast iron radiators and a high volume delivery system, installing a low volume, mod-con boiler, without using a properly sized buffer tank will give you nothing but problems. The boiler (without a tank) will constantly short cycle leading to lower efficiency and constant service problems. If it was me I would go with a Buderus GA series boiler. It has the capacity to deliver the btu's you need and it has enough water capacity to keep from cycling. In an older home there is more to consider than raw efficiency. The cost\payback ratio for very high efficiency boilers is not very good even with optimal delivery systems such as radiant or baseboard. In my own home (1800's colonial with CI radiators) I have a Buderus G125 oil fired boiler with the Logamatic control and a 40 gallon indirect for domestic hot water. The system operates flawlessly and has reduced my oil consumption by 22%. We convert about a dozen homes a year and have found out through trial and error that low mass boiler with high mass delivery are nothing but problems.

Here, I'll get my 2 cents in. If you have old cast iron radiators and a high volume delivery system, installing a low volume, mod-con boiler, without using a properly sized buffer tank will give you nothing but problems. The boiler (without a tank) will constantly short cycle leading to lower efficiency and constant service problems. If it was me I would go with a Buderus GA series boiler. It has the capacity to deliver the btu's you need and it has enough water capacity to keep from cycling. In an older home there is more to consider than raw efficiency. The cost\payback ratio for very high efficiency boilers is not very good even with optimal delivery systems such as radiant or baseboard. In my own home (1800's colonial with CI radiators) I have a Buderus G125 oil fired boiler with the Logamatic control and a 40 gallon indirect for domestic hot water. The system operates flawlessly and has reduced my oil consumption by 22%. We convert about a dozen homes a year and have found out through trial and error that low mass boiler with high mass delivery are nothing but problems.

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Can you elaborate? Adding the buffer adds mass, but you're saying mass is the problem?

Low mass boilers in ANY mass system can work well using a reverse-indirect as the buffer (TurboMax, ErgoMax, Everhot EA, etc.), making the system buffer-centric, slaving the boiler only to the buffer. All heat flows to/from the buffer while the boiler remains agnostic of zone calls. In a foggy-goggle view this sort of like the Energy Kinetics System 2000 approach with simpler controls. But the boiler can be almost arbitrarily low mass- lower than the Sys-2K steel boiler, so the cycle losses can still be quite small. In a system like this the boiler loop can be low-flow high delta-T, and the radiation flows can be whatever they need to be.

You lose some of the condensing efficiency with this approach since you won't let the buffer get much below 130F, thus the return water doesn't drop much below 120F except during extended DHW draws. But it'll never short-cycle on micro-zones, and the minimum burn cycle can be preset by tank hysteresis & boiler loop flow & output temp. But some efficiency can be gained with outdoor reset on the tank temp if the highest temp zone has a design-day temp requirement of 160F+. In high delta-T radiation/low-temp systems the return water can still be in the condensing zone though- there may be some plumbing optimizations to keep the boiler return water closer to the radiation return temps, isolating it somewhat from the mixing turbulence in the reverse-indirect when the boiler is firing. (Details will depend on they system, but it's that always the case?)

I'm familiar with a number of unbuffered high mass hydronic systems that continue to work just fine retrofitted with low mass mod-cons (sans-buffer) so I'm very curious to know more about the "...nothing but problems" issues. (Forewarned is forearmed.)

The best way is to separate the high mass delivery (piping & radiators) from the boiler loop by using an indirect tank with a coil in it. One of the dirty little secrets out there is that all of the low mass mod con boiler manufacturers are having tons of warranty problems directly related to component failure due to short cycling. System 2000 has been trying to address the problem for 20 years now by installing aquastats and controls that increase cycle time on the post purge. It does help to some degree but the problems persist. In the early days System 2000's would regularly rot out the bottom of the flue collector due to unwanted condensation.

The best way is to separate the high mass delivery (piping & radiators) from the boiler loop by using an indirect tank with a coil in it. One of the dirty little secrets out there is that all of the low mass mod con boiler manufacturers are having tons of warranty problems directly related to component failure due to short cycling. System 2000 has been trying to address the problem for 20 years now by installing aquastats and controls that increase cycle time on the post purge. It does help to some degree but the problems persist. In the early days System 2000's would regularly rot out the bottom of the flue collector due to unwanted condensation.

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None of that is surprising- there's no way a mod-con with a 4/1 turndown can reliably load-track except during the coldest days of the year (and never on micro-zoned systems.) But a low mass boiler (mod-con or not) using the "reverse indirect" buffer with the DHW in the coils (I'm assuming that's what you're referring to?) cures a lot of those ills, for a small penalty in tank standby loss (smaller than the loss incurred in a standard gas-fired HW heater.)

I'm a bit surprised that the Sys2K designers initially ran it so close to the margin that they suffered flue-condensation problems. (It's not like flue condensation issues were new-news even 20 years ago, or even 80 years ago.) It seems to me that going with very low mass boiler with condensation-tolerant heat exchangers & flues (like tankless water heaters & condensing boilers) coupled with high mass storage like an indirect makes a better paradigm than sloppy load-tracking or Rube Goldberg-style boiler heat purges. With a sufficiently low mass boiler like a small water-tube or mod-con the energy recovered in a purge is negligible.

Tankless HW heaters are basically condensation-temp tolerant copper water-tube boilers. The may not have the jacket insulation or combustion efficiency of a mod-con, but they're not priced like a mod-con either, and without low/ultra-low temp radiation the performance difference just isn't all that much. Reliability & ratings... well... that's a different matter- most tankless vendors reduce or void the warranty period when it's used for space heating (the notable exceptions being Takagi & Navien.) But for tiny heat loads that are a fraction of the peak DHW load even on design day, it can be a viable option. Coupled with a reverse-indirect they'll modulate up considerably during sustained DHW draws, since the large fraction of the heat exchange occurs at the bottom of the tank where the boiler return is. And with programmable output temps, it's not tough to design a boiler output temp, tank setpoint & boiler loop flow to deliver the design-day heat (but not more) during non DHW mode peaks to maximize the minimum burn time and prevent short cycling. (Most run about 83-86% combustion efficiencies at 120F return, @ mid-modulation.)

I'm pretty sure a tankless isn't a retrofit solution for a 4000' house in Michigan (unless it's super-insulated), but a low-mass condensation-temp tolerant RayPak copper tube boiler coupled with a reverse-indirect set up as a buffer (like a Turbomax, Everhot EA series, or ErgoMax) might be the ticket, probably for less money than a mod-con (and far more efficient than a cast-iron boiler plus standalone hot water heater):

There's a bunch of stuff that has happened in the hydronics industry that really makes you scratch your head, but then again it's all mostly driven by the all mighty dollar. AFUE ratings are for the most part a joke. Most manufacturers get higher AFUE's by drastically reducing the jacket insulation and over estimating the cycle time. The actual dollar savings between a 97% and an 89% boiler will most likely never make up for the additional cost of the equipment and installation, not to mention the addition of condensate drain's and treatment. However, like solar, there is a public perception that more is better so the manufacturers naturally jump on the band wagon. Buderus makes both low mass and conventional boilers. The company has taken a common sense approach to the issue. for new houses with either baseboard, radiant or the combination of both, a high efficiency condensing boiler may make sense when you run the numbers. Addidionally, if the equipment and radiation is properly sized, cycling can be greatly reduced or perhaps eliminated. For conversion of older systems Buderus makes the 115 and 125 series boilers. (gas models also) that are very efficient, have a good thermal mass and the jacket is highly insulated. Like most things, the homeowner is often dazzled by unscrupulous companies and shiny brochures that make promises that can not be kept.

AFUE is a nearly nonsensical test, and isn't a very good way of comparing real-world performance even when correctly sized.

But low-mass boilers reduce the standby loss factor that can dominate in cycling losses a less than full load (not just short-cycling which is even worse.) The 2-stage Lochinvar Solution series of mid-80s AFUE type low mass copper tube boilers are probably more appropriate to clappr's retrofit than the Lochinvar mod-con. Buffered with an indirect HW heater it'll probably meet/beat the performance of the mod-con + whatever hot water heater he's using. Perhaps he should discuss that with the people who handle Lochinvar.

I suspect the thermal mass of the Lochnivar is ~1/3 or less of the mid-mass Buderus 115 series, the smallest of which has ~9 gallons of water. The insulation levels on the Buderus is comparatively higher though- actual standby losses may be similar.

Getting the burner size matched to the actual load is the first critical step on the efficiency path though. You can only reduce cycling losses so far with a buffering strategy. Sizing it correctly to start with pays huge dividends: A 93-95% AFUE mod-con boiler may run at 88-90% when right-sized in the higher-temp radiation situation found in most retrofits, but will only hit 83-84% if it's 3x oversized. (Which is VERY typical.) Similar scaling factors occur with mid & standard-efficiency boilers too.

The efficiency you can get out of a system is only as good as the heating system designer. The AFUE-tested efficiency the equipment is only one factor, and not often the largest factor.

So don't let your father-in-law sell you on the stripped-down electric tank as a tempering tank in series with your standard gas HW heater as any kind of functional or efficiency equivalent to an indirect-fired HW tank. The difference in HW heating efficiency is huge. It'll cost you more than 30% more in DHW fuel use with the father-in-law approach than if you go with a low mass boiler + indirect tank. And using the indirect as a heating system buffer increases the efficiency of the heating system- the fuel savings from the heating season with that approach will likely more than pay for all of your DHW.